Shown in FIG. 1 is a section of a conventional CCD image sensor.
A conventional CCD sensor includes a n-type substrate 61, a p-type well 62 formed on the n-type substrate 61, a n+-type photo diode 63 formed on the p-type well 62 for generating signal charge, a n+-type VCCD (Vertical Charge Coupled Device) area 64 formed on the p-type well 62 spaced a certain distance from the photo diode 63 and transmitting the signal charge transmitted from the photo diode 63 to a HCCD (Horizontal Charge Coupled Device) area (not shown), a p++ layer 65 formed on the surface of the photo diode 63 for forming a electric potential barrier, a transfer gate 66 for transmitting the signal charge generated in the photo diode 63 to the VCCD area 64, a HCCD area transmitting the signal charge from the VCCD area 64 to a power amplifier(not shown), a power amplifier(not shown) for transmitting an image signal Vout after receiving the signal charge transmitted from the HCCD area, a polygate 67 for transmitting the signal charge from the VCCD area 64 to the HCCD area(not shown), an insulation layer 68 formed on the substrate 61 between the polygate 67 and the transfer gate 66 for insulating the polygate 67 from the transfer gate 66, and a p++-type channel stop area 69 for insulating between cells by forming high electric potential barriers.
In a CCD image sensor having a construction as shown in FIG. 1, the photo diode 63, on receiving light, generates a signal charge corresponding to the intensity of the light, which signal charge is transmitted to the VCCD area 64 in response to the signal applied to the transfer gate 66. This signal charge transmitted to the VCCD area 64 is transmitted to the HCCD area in response to the signal applied to the polygate 67, which is transmitted to the power amplifier in response to the signal applied to the HCCD area and, finally, output as a video signal.
However, in the event that the photo diode 63 receives a light having too high intensity, the photo diode generates excessive signal charge.
The image signal output of this excessive signal charge exhibits the blooming phenomena.
As for the methods for eliminating such excessive signal charge, there are methods in which the excessive signal charge is made to escape in a direction opposite to the VCCD area 64 through an overflow drain region formed between the channel stop area 69 and the photo diode 63, or toward the substrate by applying anti-blooming bias to the substrate 61.
In general, the blooming is controlled by applying bias to a substrate of integrated solid state image elements, which anti-blooming bias is of voltage of direct current, generally above 5 V and below 18 V.
However, for controlling a photo accumulation period of time, electrical shutter pulses having a magnitude above 20 V can be applied thereto.
Shown in FIG. 2 is a conventional manual variable anti-blooming DC bias circuit.
The conventional manual variable anti-blooming DC bias circuit has resistances R11 and R12 and a variable resistance VR11, wherein the variable resistance VR11 is adjusted to apply desired anti-blooming bias voltage to the CCD image sensor 60; the 24 V is divided by the resistances R11 and R12 and the variable resistance VR11 adjusted by a user, which is applied to the substrate SUB of the CCD image sensor 10.
Therefore, the user can prevent blooming due to the excessive light incident on the CCD image sensor by adjusting the variable resistance VR11 image sensor so as to apply appropriate anti-blooming bias to the CCD image sensor.
Referring to FIG. 2, as the 24 V power is fed through a circuitry connection, it can be more or less unstable, but with a circuit as shown in FIG. 3, a stable power can be fed to a CCD image sensor.
Shown in FIG. 3 is an example of application of the manual variable anti-blooming bias circuit shown in FIG. 2.
In the circuit shown in FIG. 3, when the stable power(15 V) is applied to the manual variable anti-blooming bias circuit, a constant voltage divided by resistances R13 and R14 is applied to a transistor Q11 at a base terminal thereon.
And the 24 V transmitted from a vertical operation part 40 is adjusted by the variable resistance VR11 and applied therefrom to a transistor Q12 at a base terminal thereon.
According to operation of the transistor Q12, a desired anti-blooming bias can be applied to an input terminal SUB of the CCD image sensor 60 through a transistor Q13.
In this time, in case the 24 V power is unstable, a permanent direct current anti-blooming bias is fed to the CCD image sensor 60 at the input terminal SUB thereon due to a continuous induction of a current flowing in the transistor by a continuous current flowing in the transistor Q11.
Accordingly, the CCD image sensor 60 transmits anti-blooming bias, an output voltage Vout based on which is transmitted to a signal processor 80 through terminal OUT and, consequently, the signal processor 80, processing the signal transmitted from the CCD image sensor 60, transmits video signals.
On the other hand, in case pulses above 15 V are transmitted from one output terminal Vsub of the vertical driving part 40, shutter pulses of 15 V DC are applied to the input terminal SUB of the CCD image sensor 60 under a condition that the 15 V voltage is set up by the diode 11, irrespective of the anti-blooming bias transmitted from a anti-blooming bias circuit 20.
Shown in FIG. 4 is potential distributions based on anti-blooming bias VOFD in accordance with the CCD image sensor of FIG. 1.
The higher the anti-blooming bias VOFD, the lower a electric potential barrier toward the substrate 61 making a signal saturation quantity ie., the quantity of signal charge which can be accumulated in the photo diode 63 less.
Shown in FIG. 5 is a graph showing relation between anti-blooming bias VOFD and smear noise in accordance with the CCD image sensor shown in FIG. 1, wherein it shows that, when anti-blooming bias VOFD becomes higher, ie., the signal saturation quantity becomes less, the smear noise increases.
Shown in FIG. 6 are graphs showing relation between the intensity of light according to anti-blooming bias and the output voltage of the CCD image sensor of FIG. 1, wherein it shows that when anti-blooming bias VOFD becomes higher, the electric potential toward the substrate is formed lower, accumulating less signal saturation quantity, so that, when little quantity of light is incident, desired output voltage Vout can not be obtained.
The lower the anti-blooming bias VOFD become, the higher the electric potential barrier toward the substrate is formed making the signal saturation quantity greater resulting to obtain the desired output voltage Vout even with little quantity of light.
Referring to FIGS. 4 to 6, in case a user adjusts the variable resistance VR11 of FIG. 2 setting the anti-blooming bias VOFD at a higher value of VOFD1 in advance, even though an excessive signal charge would be generated in the photo diode 63 when excessive light is incident thereto, it is possible to prevent blooming because the sufficient excessive signal charge is made to escape to the substrate 61.
However, it raises a problem of making the smear noise increase due to the high anti-blooming bias VOFD. on the other hand, when little quantity of light is incident thereon, even though desired quantity of signal charge be generated, due to low electric potential barrier formed toward the substrate by the high anti-blooming bias, accumulating little quantity of charge saturation, the signal charge is made to escape to the substrate creating a problem of obtaining no desired video signal.
In the meantime, in case the variable resistance VR11 of FIG. 2 is set the anti-blooming bias VOFD at a lower value of VOFD3 in advance, the anti-blooming bias VOFD becomes lower making the smear noise decrease.
However, because the anti-blooming bias is low, electric potential barrier is formed high preventing the excessive signal charge from escaping to the substrate 61, but making it transmitted to the VCCD area 64. Consequently, it raises a problem of developing blooming.